This invention relates to a novel video editing system employing the Vertical Interval Time Code (VITC) as the sole edit control code in the video tape editing process.
Computer based videotape editing systems have significantly increased the speed, convenience and quality of the video and audio editing process. The basic operating parameter of any computer-based editing system is the ability to assign each and every frame of video information a unique address. This address, termed a time-code, is then used as the identifier of any particular piece of video information to be edited. The American National Standard for the time-code signal format is the Society of Motion Picture and Television Engineer's (SMPTE) time-code signal. The SMPTE time-code signal is encoded on the record medium on a separate track from those which contain the video information. It is recorded along a longitudinal track which is usually read or written to by a stationary, dedicated head in the operating apparatus. For this reason, the SMPTE time-code has become known in the art as the longitudinal time-code or LTC. The LTC wave-form is shown in FIG. 1.
Many devices capable of reading the LTC and performing edits based on manipulation of the LTC, are known in the art. However, editing systems utilizing the LTC as the edit control code exhibit deficiencies when the record medium, e.g., videotape, is transported at very slow speeds or stopped. Video editing becomes very difficult during slow motion or still frame editing due to the fact that the LTC is recorded on a separate longitudinal track which must be moving above some minimal speed in order for a sufficient voltage to be induced at the coil of the head reading the LTC. It is an advantage to the editing process to be able to choose individual frames presented in slow motion or still frame manner, yet the inability to read the LTC proves to be a major deficiency to any such editing system in that the addresses of the frames are not available when they are viewed or edited in such a manner.
It has, therefore, been suggested in the art, as in U.S. Pat. No. 4,167,759, to create a time-code edit signal that is recorded as part of the video information on the same track as the video signal, thereby allowing the code to be read in slow motion or still frame modes in the same manner as the video signal. This edit code is referred to as a Vertical Interval Time Code (VITC) because the edit signal is inserted into two non-adjacent lines during the vertical blanking period, in order to insure that it is not presented on the screen of the viewing apparatus; line 21 generally being the first line visible on a receiver in a standard 525 line/60 field television system.
U.S. Pat. No. 4,167,759 created a VITC edit signal by reading the LTC from the longitudinal track, compressing it, and inserting it into the vertical blanking period, specifically at lines 10-21 of the vertical interval. With reference to FIG. 2, the VTR records a video signal through input terminal 20 and records the longitudinal time code on the dedicated LTC track at terminal 21. The LTC output is provided at terminal 23.
During a video edit, the video signal to be inserted is applied to the VITC inserting circuit 24. The LTC reader 27 reads the LTC address recorded on the tape of the record VTR, and provides the output to the LTC generator 25 as a pre-set signal through switch 14 so as to forward that LTC address to the VITC inserting circuit 24. The LTC address is thereby converted to VITC format and inserted into the video signal at terminal 20. Since one frame is required to read the LTC code by LTC reader 27, the LTC generator adds one to the address before conversion to VITC, thereby insuring continuity of the addresses in the edited tape. Alternatively, the VITC generator converts the forwarded address from the LTC generator and the video signal is then passed to VITC reader 26 through switch 15, which reads the VITC code and then increments it and supplies it back to VITC inserting circuit 24. A circuit for the compression and insertion of the LTC is shown in U.S. Pat. No. 4,134,130.
This version of the VITC edit code has not, however, been widely accepted in the art.
It has, therefore, been suggested by the SMPTE to provide a different version of the VITC edit signal which is not based on a compression of the LTC time-code, but rather has its own unique bit stream, as shown in FIG. 3. This version of VITC is the current specification for the waveform.
Various devices now exist which conform to this VITC edit code. For example, there are devices that can read or generate the VITC bit stream, while other devices have the capability of converting LTC to VITC or vice versa. While VITC readers and generators have found some useful enterprise, the LTC to VITC converters remain the dominant VITC-based hardware. This is so because LTC is still almost exclusively used as the edit control signal throughout the industry. The basic elements of a video editing system, i.e., the video tape recorders, signal generators and edit controllers, are manufactured for an LTC based system.
There exists, therefore, a need for a defined video tape editing system that implements the VITC edit signal as the sole edit control code. While the basic wave-form implementation has been known there exists a need for an editing system wherein the necessary, structured cooperation between the VTR's, the time code readers and generators, and the edit controllers is set forth in detail.